Summary: Many conditions that shift cells from states of nutrient utilization and growth to states of cell maintenance extend lifespan. We have carried out a systematic lifespan analysis of conditions that inhibit protein synthesis. We find that reducing the levels of ribosomal proteins, ribosomal‐protein S6 kinase or translation‐initiation factors increases the lifespan of Caenorhabditis elegans. These perturbations, as well as inhibition of the nutrient sensor target of rapamycin (TOR), which is known to increase lifespan, all increase thermal‐stress resistance. Thus inhibiting translation may extend lifespan by shifting cells to physiological states that favor maintenance and repair. Interestingly, different types of translation inhibition lead to one of two mutually exclusive outputs, one that increases lifespan and stress resistance through the transcription factor DAF‐16/FOXO, and one that increases lifespan and stress resistance independently of DAF‐16. Our findings link TOR, but not sir‐2.1, to the longevity response induced by dietary restriction (DR) in C. elegans, and they suggest that neither TOR inhibition nor DR extends lifespan simply by reducing protein synthesis.

The Myc protein binds DNA and activates transcription by mechanisms that are still unclear. We used chromatin immunoprecipitation (ChIP) to evaluate Myc-dependent changes in histone acetylation at seven target loci. Upon serum stimulation of Rat1 fibroblasts, Myc associated with chromatin, histone H4 became locally hyperacetylated, and gene expression was induced. These responses were lost or severely impaired in Myc-deficient cells, but were restored by adenoviral delivery of Myc simultaneous with mitogenic stimulation. When targeted to chromatin in the absence of mitogens, Myc directly induced H4 acetylation. In addition, Myc recruited TRRAP to chromatin, consistent with a role for this cofactor in histone acetylation. Finally, unlike serum, Myc alone was very inefficient in inducing expression of most target genes. Myc therefore governs a step, most likely H4 acetylation, that is required but not sufficient for transcriptional activation. We propose that Myc acts as a permissive factor, allowing additional signals to activate target promoters.

The transcription factor MYC binds specific DNA sites in cellular chromatin and induces the acetylation of histones H3 and H4. However, the histone acetyltransferases (HATs) that are responsible for these modifications have not yet been identified. MYC associates with TRRAP, a subunit of distinct macromolecular complexes that contain the HATs GCN5/PCAF or TIP60. Although the association of MYC with GCN5 has been shown, its interaction with TIP60 has never been analysed. Here, we show that MYC associates with TIP60 and recruits it to chromatin in vivo with four other components of the TIP60 complex: TRRAP, p400, TIP48 and TIP49. Overexpression of enzymatically inactive TIP60 delays the MYC‐induced acetylation of histone H4, and also reduces the level of MYC binding to chromatin. Thus, the TIP60 HAT complex is recruited to MYC‐target genes and, probably with other other HATs, contributes to histone acetylation in response to mitogenic signals.

TheCaenorhabditis elegansNuclear Hormone Receptor NHR-49 coordinates expression of fatty acid (FA) metabolic genes during periods of feeding and in response to fasting. Here we report the identification of MDT-15, a subunit of theC. elegansMediator complex, as an NHR-49-interacting protein and transcriptional coactivator. Knockdown ofmdt-15by RNA interference (RNAi) prevented fasting-induced mRNA accumulation of NHR-49 targets in vivo, and fasting-independent expression of other NHR-49 target genes, including two FA-Δ9-desaturases (fat-5,fat-7). Interestingly,mdt-15RNAi affected additional FA-metabolism genes (including the third FA-Δ9-desaturase,fat-6) that are regulated independently of NHR-49, suggesting that distinct unidentified regulatory factors also recruit MDT-15 to selectively modulate metabolic gene expression. The deregulation of FA-Δ9-desaturases by knockdown ofmdt-15correlated with dramatically decreased levels of unsaturated FAs and multiple deleterious phenotypes (short life span, sterility, uncoordinated locomotion, and morphological defects). Importantly, dietary addition of specific polyunsaturated FAs partially suppressed these pleiotropic phenotypes. Thus, failure to properly govern FA-Δ9-desaturation contributed to decreased nematode viability. Our findings imply that a single subunit of the Mediator complex, MDT-15, integrates the activities of several distinct regulatory factors to coordinate metabolic and hormonal regulation of FA metabolism.

E2F proteins can either activate or repress transcription. Following mitogenic stimulation, repressive E2F4-p130-histone deacetylase complexes dissociate from, while activating species (E2F1, -2, and -3) associate with, target promoters. Histones H3 and H4 simultaneously become hyperacetylated, but it remains unclear whether this is a prerequisite or a consequence of E2F binding. Here, we show that activating E2F species are required for hyperacetylation of target chromatin in human cells. Overexpression of a dominant-negative (DN) E2F1 mutant in serum-stimulated T98G cells blocked all E2F binding, H4 acetylation, and, albeit partially, H3 acetylation. Target gene activation and S-phase entry were also blocked by DN E2F1. Conversely, ectopic activation of E2F1 rapidly induced H3 and H4 acetylation, demonstrating a direct role for E2F in these events. E2F1 was previously shown to bind the histone acetyltransferases (HATs) p300/CBP and PCAF/GCN5. In our hands, ectopically expressed E2F1 also bound the unrelated HAT Tip60 and induced recruitment of five subunits of the Tip60 complex (Tip60, TRRAP, p400, Tip48, and Tip49) to target promoters in vivo. Moreover, E2F-dependent recruitment of Tip60 to chromatin occurred in late G₁ following serum stimulation. We speculate that the activities of multiple HAT complexes account for E2F-dependent acetylation, transcription, and S-phase entry.

Significance: The unfolded protein response of the endoplasmic reticulum (UPR ^ER ) is induced by proteotoxic conditions. Lipid disequilibrium also activates the UPR ^ER , but whether this activation is accompanied by disturbed proteostasis in vivo remains controversial. In this study, we show that in the nematode Caenorhabditis elegans compromised fatty acid desaturation and reduced phosphatidylcholine production activate the UPR ^ER without overt proteostatic imbalance, as assessed by molecular, pharmacological, and genetic analyses. This finding suggests that membrane composition is a direct input able to activate UPR ^ER signaling even when proteostasis in the ER is largely intact. The activation of the UPR ^ER by independent inputs may reflect the central role of the ER in both lipid and protein biosynthesis.